The present invention relates generally to the field of locomotives and their control. More particularly, the invention relates to techniques for maximizing the tractive effort or braking effort of a locomotive and minimizing the resonant torsional vibration per axle to improve overall performance.
Locomotives and transit vehicles as well as other large traction vehicles are commonly powered by electric traction motors coupled in driving relationship to one or more axles of the vehicle. Such locomotives and transit vehicles typically have at least four axle-wheel sets per vehicle with each axle-wheel set being contacted via suitable gearing to the shaft of a separate electric motor commonly referred to as a traction motor. In a motoring mode of operation, the traction motors are supplied with electric current from a controllable source of electric power such as an engine-driven traction alternator. The traction motors apply torque to the vehicle wheels, which in turn exert tangential force or tractive effort on the surface such as the parallel steel rails of a railroad track on which the vehicle is traveling, and thereby propel the vehicle in a desired direction along the right of way. In another instance, in an electrical braking mode of operation, the motors serve as axle-driven electrical generators. Torque applied to the shafts of the axle-wheel sets in such an instance by their respective associated axle-wheel sets then exert braking effort on the surface, thereby retarding or slowing the vehicle's motion. In either case, good adhesion between each wheel and the surface is required for efficient operation of the vehicle.
Maximum tractive effort or braking effort is obtained if each powered wheel of the vehicle is rotating at such an angular velocity that its actual peripheral speed is slightly higher (in case of motoring) or slightly lower (in case of braking) than the actual speed of the vehicle. The linear speed at which the vehicle is traveling is usually referred to in literature as ground speed or track speed and the difference between wheel speed and ground speed is referred to as slip speed or creep. There is usually a relatively low limit on the value of slip speed at which peak tractive effort or braking effort is realized. This value, commonly known as optimum creep is a variable that depends on ground speed and rail conditions. Operation of any or all wheels away from the optimum creep, for instance, at too small a creep value or too large a creep value, may result in a reduction or loss of wheel-to-rail adhesion. Likewise, if the wheel-to-rail adhesion tends to be reduced or lost, some or all the vehicle wheels may slip excessively, i.e., the actual slip speed or creep may be greater than the optimum creep. Such a wheel slip condition, which is characterized in the motoring mode by one or more slipping axle-wheel sets and in the braking mode by one or more sliding or skidding axle-wheel sets, can cause accelerated wheel wear, rail damage, high mechanical stresses in the drive components of the propulsion system, and an undesirable decrease of tractive (or braking) effort. Accordingly, it is desirable to control the allowable creep of all the wheels to maximize the total traction performance. There are many difficulties associated with determining an optimal creep set point for peak adhesion. Creep optimization requires measurement/estimation of wheel tractive effort and wheel creep. Direct measurement of the wheel tractive effort using strain gages for instance, is expensive, requiring significant changes to the wheels. Moreover, the sensors used for direct measurement of wheel tractive effort are also prone to noise. Some of the prior art involve estimation of the wheel tractive effort based on motor torque measurement by a simplistic method using the gear transmission ratio and the wheel radius; see, e.g., U.S. Pat. No. 6,208,097 issued on Mar. 27, 2001 to General Electric Company (hereby incorporated into the present disclosure by reference). Such a method is also error prone owing to approximation of the dynamics of the drive train and related process noise. This method especially loses it validity when the drive train resonates at its natural mode of vibration. One such exemplary situation occurs when inter-axle dynamics owing to mechanical coupling between the platform, all the trucks and all the axle-wheel sets, become pronounced. This will be the case when, for instance, the pitching and rolling modes of the trucks and platform of the locomotive resonate resulting in significant weight-shift effects and associated wheel normal force variations. The resonance may typically manifest in significant oscillations in the motor torque adversely affecting the accuracy of torque maximization methods disclosed in prior art.
Wheel creep requires measurement/estimation of wheel speed and ground speed. Adhesion control systems and methods found in prior art describe determination of wheel speed from the speed of the motor shaft in a well-known manner using the gear transmission ratio. The relationship between the motor speed and wheel speed is typically assumed to be algebraic and the ratio of the motor speed to the average wheel speed of the axle is taken to be equal to the gear ratio. This method of determination of wheel speed is simplistic and is prone to errors owing to errors in motor speed measurement and also owing to approximation of the dynamics of the drive train while estimating wheel speed. The latter especially is the case when the drive train resonates at its natural mode of vibration resulting in a difference in speeds of the two wheels of the axle-wheel set.
Another factor affecting traction performance is the level of torsional resonant vibration in the mechanical drive train, which comprises the axle and its associated two wheels, the motor to the axle gearbox, the traction motor and the traction motor drive. In particular, during operation in certain regions of the adhesion characteristic curve, the mechanical drive train might experience a net negative damping, which produces severe vibration levels at natural frequencies or vibration modes of the system. As is well known, an adhesion characteristic curve graphically represents the coefficient of adhesion versus percentage creep. At zero percent creep, maximum damping on the mechanical system is represented. As the percent creep level increases in motoring (or decreases in braking) in the portion of the characteristic curve to the left of its peak in motoring (or right of its peak in braking), the damping effect on the mechanical system decreases to a value of zero at the peak. For increasing percent creep values to the right of the peak in motoring (or left of the peak in braking), the damping provided to the mechanical system becomes a large negative number.
The natural frequencies or vibration modes of a system are a function of the drive train component materials and geometries that vary slightly over the life of a vehicle due to wear and tear. Dependent on the magnitude and duration of the vibration periods, the drive train may be damaged. Accordingly, it is desirable to minimize torsional resonant vibration in order to maximize traction performance. Prior art involves frequency response analysis of estimated torque feedback of each traction motor. Since such a method is based only on one signal, there is scope for improvement in accuracy and reliability by considering many more representative signals.
These issues in the measurement/estimation of wheel speed values and wheel tractive effort values result in an adhesion optimization system that usually operates sub-optimally. Accordingly, it is desirable to obtain the best possible estimates/measurements of wheel speeds, wheel tractive efforts and extent of torsional vibration, accounting for the influence of various dynamics of the drive train and also the influence of inter-axle dynamics pertaining to various suspensions and the mechanical coupling between the locomotive platform, all the trucks and all the axle-wheel sets.